Eru's intervention in history that changed Arda from flat to round, making the West inaccessible to Men

The Changing of the World was the event in which Arda was transformed from a flat world into a round planet. It occurred in S.A.3319[1] when Eru destroyed the island of Númenor due to the deeds of its people.

When the NúmenóreanKingAr-Pharazôn attacked Valinor, the Valar relinquished their dominion over Arda and called upon Eru for help. Eru responded by creating an enormous rift between Númenor and the Blessed Realm, into which Ar-Pharazôn's fleet and, ultimately, his land were swept. Meanwhile, the flat world bent back upon itself and met at this rift. The final result was a round planet of which Aman and Tol Eressëa were no longer a physical part. After the Changing of the World, only the Elves could find the Straight Way and reach the ancient West by ship.[2]

In Tolkien's latest writings published in the later volumes of The History of Middle-earth, Arda was a round world from its beginning, and the idea of a flat world was instead an invention of the Númenóreans. Thus, in this alternate account, the world would not need to be bent at the time of Númenor's downfall and Aman's removal from the physical world.[3]